1. Field of the Invention
The present invention relates to a power steering system for generating an auxiliary steering force for the steering of a vehicle such as a four-wheel-drive car or the like.
2. Description of the Related Art
FIG. 16 shows an example of a power steering system. The power steering system as shown in FIG. 16 is disclosed in Japanese Patent Application, First Publication, No. Hei 9-84300, in which output shaft 2 of driving unit (i.e., motor) 1 for generating the above auxiliary steering force and input shaft 3 coupled to the steering unit of a vehicle are coupled via torque limiter 4. This torque limiter 4 comprises cylindrical limiter cover (or case) 5 having a bottom, fixed in a manner such that the cover is rotationally driven together with the output shaft 2, similar cylindrical cover 6 having a bottom, which covers from the opening side to the outer surface of the limiter cover 5, limiter plate (or boss) 7 which is arranged to be relatively rotatable with respect to limiter cover 5 and is coupled to the input shaft 3, and friction plate 9 which is arranged between cover 6 and limiter plate 7 via forcing member (or spring member) 8 so as to be driven together with the limiter cover 5 and which is relatively rotatable with respect to the limiter plate 7.
The above cover 6 is attached on limiter cover 5 after limiter plate 7, forcing member 8, and friction plate 9 are set in the limiter cover 5. The cover 6 is fixed by folding down and clamping the peripheral edge of its opening side, and forcing member 8 comes in contact with the bottom of the cover 6, by which pressing force by this forcing member 8 is supported. In the power steering system having the above-described structure, during ordinary steering, auxiliary steering force generated from driving unit 1 is properly transmitted from output shaft 2 via torque limiter 4 to input shaft 3 by frictional force generated between limiter cover 5, friction plate 9, and limiter plate 7 caused by pressing force of forcing member 8. In a case in which impact force is transmitted from the wheel side of the steering unit, a slip is generated between limiter cover 5, friction plate 9, and limiter plate 7 of torque limiter 4, and impact torque is absorbed; thus, it is possible to prevent excessive torque from acting on output shaft 2 of driving unit 1.
In torque limiter 4 in such a power steering system, a predetermined frictional force is generated between limiter cover 5, friction plate 9, and limiter plate 7, as described above; thus, it is necessary to ensure sufficient contact areas between these members. Accordingly, it is inevitable that the outer diameter of torque limiter 4 is considerably large. However, in a torque limiter having such a large outer diameter, which is rotated together with output shaft 2 and input shaft 3, the moment of inertia of this torque limiter 4 also becomes large, and there occur problems in that the driving force necessary for rotationally driving output and input shafts 2 and 3 is increased, and follow-up capability between rotation of input shaft 3 and rotation of output shaft 2 is degraded. In particular, in the above conventional power steering system, cover 6 is arranged to cover the outer surface of limiter cover 5. Therefore, the outer diameter of torque limiter 4 is further increased and the moment of inertia is also increased; thus, the above-mentioned problems are assumed to be very substantial.
Also in torque limiter 4 in the above power steering system, it is required that during ordinary steering, auxiliary steering force be properly transmitted from output shaft 2 to input shaft 3, while when impact force acts thereon, impact torque is securely absorbed, as described above. Therefore, pressing force caused by the forcing member 8 must be strictly controlled so as to set the friction force between limiter cover 5, friction plate 9, and limiter plate 7 to be in a predetermined range. However, in the above conventional power steering system, the pressing force caused by the forcing member 8 is supported by cover 6 which covers limiter cover 5, and the opening of this cover 6 is simply clamped and fixed to limiter cover 5. Therefore, the support position of forcing member 8 is liable to change according to clamping conditions, and there is also a possibility that cover 6 will shift due to spring-back and deformation of the clamped opening of cover 6, by which support of forcing member 8 becomes unstable. According to the above possibilities, there occurs another problem in that predetermined frictional force is not provided between limiter cover 5, friction plate 9, and limiter plate 7.